Your compost has to be wet, but not too wet. Although moisture and aeration are often discussed as separate issues in composting, your actions to manage water and air in the compost pile are closely linked.
Billions of pore spaces surround the organic particles in your compost. Pores allow air and water to circulate through the compost ingredients. If there's insufficient moisture, the decomposer organisms close up shop. On the other hand, if pores are flooded with water, airflow is hindered, and you're stuck with a smelly anaerobic (without air) compost pile to deal with.
Your goal is to balance moisture and air levels to optimize conditions for the decomposers, thereby maximizing your composting efforts.
The organisms that help with the breakdown of your organic waste require moisture to survive. Most of them perform their decomposing magic in ultra-thin films of water on the surface of organic particles. When your pile's moisture level drops below 35 to 40 percent and materials dry out, most of the creatures die or go dormant.
The ideal moisture content for your compost pile is 40 to 60 percent by weight. Nope, there's no need to weigh anything! An easy method to judge moisture content is to squeeze a few handfuls of materials from different areas of the pile. Everything should feel damp, like a wrung-out sponge. If it doesn't, it's time to add water.
You can help preserve existing moisture in your open compost pile by covering it with a tarp.
On the other hand, soggy materials handicap your composting. Moisture content above 65 to 70 percent blocks air flow and develops into stinky anaerobic conditions. Nutrients also leach out of overly wet compost piles. If you can squeeze more than a drop or two of water out of a handful of ingredients, the pile is too wet.
Pore spaces in the pile provide essential oxygen for the survival of composting organisms. Pores also allow for the escape of carbon dioxide, which is a byproduct of their decomposing efforts. Adequate aeration also helps maintain high temperatures, which produce faster rates of decomposition and kill weed seeds and pathogens.
If you live in extremely rainy regions, covering your pile helps prevent it from turning soggy during a deluge.
Fine-tune your pile's moisture and air levels by:
Turning the organic matter to introduce more air and/or dry out wet materials. A properly aerated pile has no bad odors. If it smells, it's likely too wet and needs to be turned!
Adding dry carbon materials, such as leaves, straw, or sawdust, to soak up excess moisture.
Rewetting materials if they dry out, usually at the same time you turn the pile.